1. Field of the Invention
The present invention relates in general to a method and apparatus for detecting anomalous shadows, and in particular to a method and apparatus for detecting anomalous shadows from a difference image, which is a comparative image representing the difference between two radiation images.
2. Description of the Related Art
In a variety of fields, the comparative reading of two or more images of the same subject obtained at different photographing times to detect the difference between the aforementioned two or more images is performed, and an examination, etc., of the subject of photographing is performed based on the detected difference.
In the field of industrial product manufacturing, for example, an image obtained of a product in the new state is compared to an image of the same product that has been obtained after said product has been subjected to a durability test, and by focusing mainly on the area in which the largest difference between the two images appears, the portion thereof that should be improved to improve the durability of the product is determined. Further, in the field of medicine, by comparatively reading a plurality of radiation images, which have been obtained in a temporal series, of a diseased portion of a patient, a physician can discern the course of the disease and the effectiveness of the treatment can be discerned and determine the course of future treatment.
In this way, the comparative reading of two or more images obtained of the same subject is performed in a variety of fields on a daily basis, and in order to perform this comparative reading, there are cases for which these two or more images are outputted on an image display apparatus or the like. That is to say, the image is converted to luminosity and density signals, and then displayed on an image display apparatus or the like, or outputted by a printer to a film or other such medium.
For cases in which the two or more images that are the objects of comparative reading are to be displayed, etc., although the general practice is simply to display said images an operator performing a comparative reading thereof is the area showing the difference between said images. However, in a case in which, for example, two images that are to become the objects of comparative reading as described above are simply displayed next to each other, as the size of the difference between becomes smaller the detection thereof becomes more difficult; therefore, there is a demand for an improvement in the performance of comparative reading.
Therefore, interimage computational processes, starting with a subtraction process between the pixels corresponding to each of the respective two or more images that are the objects of comparative reading, is performed, and the difference therebetween is enhanced. Because it becomes possible to accurately render the difference between the images visually discernable, by enhancing only the difference between the images in this way, to an operator performing the comparative reading, it is possible to prevent the overlooking of the difference between the images, particularly in the medical field with respect to the course of a disease or a diseased portion requiring treatment. For example, on a difference image (a so-called temporal subtraction image) obtained by the above-described interimage processes and representing the difference between two chest x-ray images obtained in a temporal series, it is possible to display even small, early-stage lung tumors, which are substantially 100% treatable, and the extraordinary effectiveness for the use in diagnosis is widely acknowledged.
Further, a method wherein, by adding the above-described subtraction image to a temporal image, the change that has occurred in a diseased portion becomes easier to discern visually, has also been proposed (refer to Japanese unexamined Patent Publication No. 8(1996)-77329).
However, although the process of selecting the diseased portion on the difference image is performed visually and manually by an operator performing a comparative reading as required, the accuracy of said selection is dependent upon the experience level and ability of the operator, and it is not a foregone conclusion that said selection will always be an objectively viable selection. In a case, for example, in which the objective of radiation images that are to be the objects of comparative reading is the detection of lung tumors, it is necessary to extract the small, faint suspected anomalous shadows that are one indicator of cancerous growths; however, it is not guaranteed that those shadows will be accurately selected. Therefore, there is a demand for a method and apparatus capable of accurately detecting the anomalous shadows that is not dependent upon the skill level of the operator.
In order to respond to this demand, CADM (Computer Aided Diagnosis of Medical images), which has an objective of accurately and automatically detecting the suspected anomalous shadows by employing computational processes, has been advancing in recent years.
That is to say, CADM technology, by performing the detection of the above-described suspected anomalous shadows based on performing a detection process for detecting the density distribution and shape characteristics thereof utilizing a computer, is a technology that automatically detects the suspected anomalous shadows. A variety of CADM technologies for application to chest X-ray images and which provide for a high probability of detecting diseased portions such as lung tumors have been proposed (refer to, for example, Japanese Unexamined Patent Publication No. 9(1997)-185714).
Here, the method proposed in the aforementioned Japanese unexamined Patent Publication No. 9(1997)-185714 is a method for applying CADM technology to an original image and an energy subtraction image (that is, the difference image between a signal enhanced image and a signal suppressed image) for detecting suspected anomalous shadows. However, there is a possibility that the early-stage lung tumors in an original image or an energy subtraction image will be overlooked even by an experienced diagnostician; even by applying CADM technology, it has been nearly impossible to extract the suspected anomalous shadows of diseased portions such as early-stage lung tumors, which are easily overlooked.
Therefore, applying CADM technology to the above-described difference image has been considered. Here, when the above-described difference image is to be formed, although it is necessary to match the positions of two images, even if position matching is performed, slight misalignments occur at the outlines of the structuring elements (positional misalignments), and artifacts due to these positional misalignments appear in the difference image. In particular, if the subject of photographing is a living-tissue subject, the 3-dimensional misalignments due to fluctuations in the angle between the irradiation direction of the radiation and the body axis of the subject of photographing, the angle of entry of the radiation relative to the orientation of the subject of photographing, etc., that appear in a 2-dimensionally displayed image are exceedingly difficult to correct (position matching) therefore, artifacts remain in the difference image.
Essentially, because only the actual difference portion representing the change, etc. of the diseased portion appears if artifacts do not occur in a difference image, by detecting the image portion that appears in a difference image, the size, etc. of the position of the diseased portion can be efficiently determined. However, if artifacts appear in the difference image, the above-described actual difference between the images does not stand out within the difference image due to the presence of the artifacts, and even if CADM technology is applied, there is a fear that it will not be possible to detect the difference.